1. Field of the Invention
The present invention relates to a solid electrolyte, which is used as a key component in a solid oxide fuel cell (SOFC). More particularly, the present invention relates to a highly ionic conductive zirconia electrolyte, which includes a multicomponent rare earth oxide stabilizer so that an ionic conductivity drop rate may be mitigated, and to a unit cell for a high-efficiency SOFC using the same.
2. Description of the Related Art
A SOFC, which includes as a core part, a ceramic unit cell comprising ceramic electrodes and an electrolyte, may operate at the highest temperature among various fuel cells. Furthermore, a SOFC exhibits high energy conversion efficiency and is able to operate a gas turbine or a micro gas turbine in two steps using hot steam discharged via high-temperature operation, thereby enabling construction of a high-efficiency combined cycle power generation system. In addition, a SOFC is advantageous because of high fuel selectivity, and thus can employ any hydrocarbon-based fuel, as well as hydrogen fuel gas. Hence, the U.S. Department of Energy (DOE) is sponsoring the research of MW class integrated gasification fuel cell (IGFC) technology.
FIG. 1 is a graph illustrating the power density of unit cells depending on the kind and ionic conductivity of the electrolyte used therein. When a SOFC operates at the same temperature, output characteristics of the SOFC, namely, power density (W/cm2), may be closely related with the ionic conductivity of the electrolyte. Also, when the same electrolyte is used, the ionic conductivity of the electrolyte is increased exponentially in proportion to an increase in the operating temperature, ultimately enhancing the power density of the SOFC. However, an increase in the operating temperature may deteriorate high-temperature stability and long-term durability of a SOFC.
A conventional zirconia electrolyte (8YSZ electrolyte) retains ionic conductivity as high as about 0.04 S/cm at an operating temperature of 800° C. However, when the operating temperature is lower, ionic conductivity may drastically drop, and thus limitations are imposed on applying such an electrolyte to medium/low temperature operating SOFCs.
For this reason, in developed countries that are leading the commercialization of SOFC technology, the development of novel electrolyte materials having high oxygen ionic conductivity even at medium/low operating temperatures has emerged as an important research area. However, newly developed alternative electrolyte materials may have their own inherent drawbacks, and thus continuous material improvement is required.
Meanwhile, a zirconia-based electrolyte has ionic conductivity varying depending on the kind and amount of stabilizer and the ionic radius of stabilizer. In particular, as a size difference between the radius of zirconium ion (Zr4+) as a main lattice material and the cation radius of rare earth (Re) oxide as a stabilizer is smaller, the oxygen ionic conductivity of the electrolyte may increase. Hence, scandia (Sc2O3) stabilized zirconia exhibits the highest ionic conductivity. Especially, 11 mol % scandia stabilized zirconia (hereinafter, abbreviated to “11ScSZ”) does not drop in ionic conductivity even after long-term use and is thus regarded as an ideal electrolyte material. However, the 11ScSZ electrolyte is disadvantageous in terms of phase transition to a monoclinic structure at a temperature lower than about 630° C. and to a cubic structure in the higher temperature range.
With the goal of solving such phase transition problems, thorough research and development has been carried out. Specifically, Toho Gas, Japan has developed and commercialized a novel electrolyte in which a portion of any component of 11ScSZ electrolyte is substituted with ceria (CeO2) and thus which is stabilized into a cubic structure in the temperature range from room temperature to high temperature. As disclosed in JP 2008-305804 A by Toho Gas, the electrolyte composition is composed of 8.5˜15 mol % of scandia and 0.5˜2.5 mol % of yttria and/or ceria under the condition that the total amount of scandia and yttria and/or ceria is formulated to 9˜15 mol %. A commercially available electrolyte product is 10Sc1CeSZ (10 mol % Sc2O3-1 mol % CeO2-89 mol % ZrO2).
The 10Sc1CeSZ electrolyte has no known phase transition problems, but new drawbacks may appear. Specifically, a unit cell using the 10Sc1CeSZ electrolyte is continuously decreased in power density (W/cm2) with operating time.
The cause of such problems, which has not been clarified to date, is ascertained by Kceracell Co. Ltd to be due to newly introduced CeO2. The 10Sc1CeSZ electrolyte is present as a white sintered body after sintering at 1400° C. or more, and maintains the same color even when exposed to air. However, in the case where this electrolyte is exposed to a reducing atmosphere including hydrogen, it is discolored to red. As a typical ceria-based electrolyte for SOFCs, Gd0.1Ce0.9O1.95 exhibits the same discoloration. The ceria-based electrolytes show high oxygen ionic conductivity compared to the 11ScSZ electrolyte, but are problematic because Ce4+ is reduced to Ce3+ upon exposure to a reducing atmosphere (hydrogen fuel gas atmosphere) at a temperature equal to or higher than 650° C. Hence, such characteristics make it difficult to utilize such electrolytes as actual electrolytes that are simultaneously exposed to air (oxidation atmosphere) and fuel (reducing atmosphere) gas.
Consequently, instability of the 10Sc1CeSZ electrolyte in a reducing atmosphere is considered to cause a continuous decrease in power of the unit cell. Hence, there is a need for technical development for providing a novel zirconia electrolyte in the art where scandia stabilized zirconia is stabilized into a cubic crystal structure and also which is improved in stability in a reducing atmosphere.